In order to maximize the information content transmitted over a given spectral bandwidth (often measured in bits per Hz of spectral bandwidth), polarization multiplexing (referred to as “PolMux”) is being increasingly used with new transmission formats. The underlying idea is that the spectral efficiency (conveniently measured in units of bits/Hz) can be effectively doubled by employing two orthogonally polarized data-carrying signals, typically having the same symbol rate, sharing the same optical signal bandwidth. Normally, these two orthogonally polarized signals are transmitted with approximately the same power, rendering the total resultant light effectively unpolarized as seen from a test-and-measurement instrument having low electronic detection bandwidth, such as is the case with most commercial Optical Spectrum Analyzers (OSA).
The Optical Signal-to-Noise Ratio (OSNR) is a direct indicator of the quality of signal carried by an optical telecommunication link. Under normal and proper operating conditions, the OSNR of an optical communication link is typically high, often in excess of 15 dB or 20 dB, or even greater. The dominant component of the noise in an optical communication link is typically unpolarized Amplified Spontaneous Emission (ASE), which is a broadband noise source contributed by the optical amplifiers in the link. In general, the ASE may be considered to be spectrally uniform across the small wavelength range spanning the optical signal bandwidth.
Some methods exist for characterizing ASE noise on optical telecommunication signals based on an in-band analysis of the optical signal. Such methods include methods referred to as “polarization-nulling” methods (see J. H. Lee et al., “OSNR Monitoring Technique Using Polarization-Nulling Method”, IEEE Photonics Technology Letters, Vol. 13, No. 1, January 2001) as well as variants thereof, and the methods referred to as “differential polarization response” methods (see International Patent Application Publication WO 2008/122123 A1 to Gariepy et al.; and WO 2011/020195 A1 to He et al., both applications being commonly owned by the Applicant). However, such methods are based on the assumption that the signal is generally highly polarized, an assumption that is not valid in the case of polarization-multiplexed signals.
For the case of most polarization-multiplexed signals, the “signal”, as detected on a photodiode having low bandwidth electronics for instance, appears unpolarized, and hence, these above-mentioned in-band OSNR measurement methods cannot be used to reliably provide the OSNR measurement.
In order to measure the noise level or the OSNR on polarization-multiplexed signals, system manufacturers and operators currently have to resort to turning off the signal of interest at the transmitter in order to measure the noise level and thereby determine the OSNR. A first limitation of this method is that it is highly disruptive and normally unsuitable for use in a “live” (i.e. carrying commercial payload) network, and is completely unsuitable for “monitoring” applications. Secondly, this method is predicated upon certain assumptions about the noise variations that occur when the signal-under-test is extinguished in order that its OSNR can be determined.
A method of measuring the noise level on polarization-multiplexed signals using an acquired optical spectrum trace of the signal was proposed by Gariepy et al. (see International Patent Application Publication WO 2011/097734 A1, commonly owned by the Applicant). This method is based on the knowledge of the spectral shape of the data-carrying signal contribution provided by a reference signal. Based on this knowledge, the signal contribution and the ASE noise contribution, that otherwise appear as being merged together on the optical spectrum trace, may be mathematically discriminated from one another. Knowledge of the spectral shape of the signal contribution may come from the acquisition of a reference signal taken, for example, at a different point, generally upstream, along the same optical communication link (i.e. the reference signal originates from the same optical transmitter), where either the OSNR is known or where the signal can be considered free of ASE noise. This method assumes that, within the optical signal bandwidth, the spectral shape of the signal does not significantly change as it propagates along the communication link. The signal contribution of such a reference signal is then spectrally representative of the signal contribution of the signal-under-test. The fact that this method requires that a reference signal be measured may be considered as a drawback for some applications.
The “instantaneous” degree of polarization (DOP) of the coherent sum of the orthogonally-polarized data-carrying contributions of a PolMux signal is actually very high (normally approaching 100% when impairments such as chromatic dispersion are minimal), provided that detection is carried out with a sufficiently high electronic bandwidth. Ideally, this bandwidth is of the order of the signal symbol rate (“baud”). In a recent publication, Saida et al. (“In-band OSNR Monitor for DP-QPSK Signal with High-speed Integrated Stokes Polarimeter”, European Conference on Optical Communications, Paper Th.2.A.2, September 2012) describe means to characterize OSNR of a commonly-deployed PolMux modulation format (i.e. DP-QPSK) using a compact polarimeter having such a high detection bandwidth. This approach would likely not be commercially viable for use as a widely-deployed DWDM signal monitor, due to the inherently high cost of employing electronics having roughly the same bandwidth as the signal symbol rate.
There is therefore a need for a method suitable for measuring in-band noise parameters such as the OSNR of polarization-multiplexed signals, where the method may employ detection electronics having a bandwidth one or more orders of magnitude less than the signal symbol rate.